I'd always thought of Mahna Mahna (that's apparently how it's spelled) as my favorite Sesame Street sketch, but it turns out to be from the Muppet Show. I actually hadn't remember very much of it (except the audio) over the years, so it was nice to see it again. Now I need to find out which episode this is from, so I can see if it's one of the ones I taped a while back when Hallmark was showing them.
It takes a while to load up even on a fast connection, but for some reason it insisted on playing very erratically while I was still loading it, so you might want to pause it until it's fully downloaded if this happens to you.
[Hat tip: In Hoc Signo Vinces]